A polarizing film used in an image display device (especially, a liquid crystal display) needs to have both high transmittance and high polarizing efficiency to provide an image bright and good in color reproducibility. This type polarizing film has been heretofore produced in such a manner that a dichroic substance such as dichroic iodine or dichroic dye is oriented on a polyvinyl alcohol (PVA) film.
Improvement in optical characteristic and improvement in in-plane uniformity as well as increase in size of a polarizing plate used in a liquid crystal display have been required recently with the advance of increase in size of the liquid crystal display, improvement in function and improvement in luminance. It is necessary to uniaxially stretch a wide raw film evenly to obtain a large-size polarizing plate. This is however a very difficult process in practice. Optical characteristic as well as in-plane uniformity shows a tendency to worsening. When the in-plane optical characteristic is not uniform, display irregularity occurs in a formed image display device to cause a remarkable problem. A wet stretching method and a dry stretching method are mainly known as a stretching method for aligning the dichroic substance. As a method for obtaining a polarizing film having a high orientation degree from a wide raw film, several proposals have been made mainly on the dry stretching method (e.g., see Reference 1).
[Reference 1]
JP A 2002-326278 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application)
In the aforementioned method, there is however a problem that in-plane uniformity is lowered as well as the uniaxial characteristic of the oriented film is lowered. An object of the present invention is to provide a method for producing a polarizing film uniform in in-plane optical characteristic and high in orientation degree even in the case where a wide raw film is used. Another object of the present invention is to provide a polarizing film produced by the production method, an optical film as a laminate of the polarizing film and another optical layer, and an image display device using the polarizing film or the optical film.